Ultrasound is used routinely to examine the interior of mechanical components or the human body. Generally, the pulse echo technique is used. This technique represents the most important application of ultrasound in medical diagnosis.
In the pulse echo technique, a short pulse of ultrasonic energy from an ultrasonic transducer is transmitted into the body. The frequency range is typically between 1 Megahertz and 30 Megahertz and generally in the range of 1 Megahertz to 8 Megahertz. Discontinuities in the acoustic impedance (the product of the velocity of sound and the density) of the tissue in the path of the ultrasound reflect some of the energy, thus forming an echo. The time taken for the echo to be returned to the transmitting transducer following the transmission of the initial pulse is a measure of the distance of the discontinuity from the transducer. In the pulse echo technique, the same ultrasonic transducer is used for both transmitting and receiving.
In medical applications, the difference in acoustic impedance between adjacent layers of tissues is small. Thus, the echoes reflected by the boundaries between the tissues are faint. However, these echoes may be detected by sensitive receivers, amplified and ultimately displayed on a television monitor as a line of varying intensity. The intensity may be made to depend on the strength of the received echoes. The transducer is scanned across the body as pulses are repeatedly generated and received, so as to generate a cross-sectional image of the body which is ultimately displayed on the monitor. Such ultrasonic "echography" has proved to be of value as a diagnostic aid in soft tissue areas such as the abdomen, pregnant uterus, eye, breast, brain, kidney, liver and heart.
In ultrasonic diagnostic equipment the transducer is the eye of the system. It is the only interface between the electronic system and the patient and is therefore one of the most important components of the system. In the pulse echo technique, the ultrasonic transducer transforms an applied voltage to ultrasonic energy and transforms echoes in the form of returning ultrasonic energy back to an electrical signal.
The basic component of an ultrasonic transducer used in the pulse echo technique is a piezoelectric element. When a voltage is applied to a piezoelectric element, the surfaces of the element move in synchronism with the applied voltage. Particles of a medium making contact with the surfaces are set in motion and an ultrasonic wave is propagated.
A piezoelectric transducer element operates at maximum efficiency when the frequency of the applied voltage is the same as its resonant frequency. The resonant frequency is inversely proportional to the thickness of the piezoelectric element.
The construction of a transducer is specialized for different applications. In general, for pulse echo applications, the transducer is designed to maximize the energy of the wave propagated from the front of the transducer and to minimize the energy loss from the back of the transducer.
When an ultrasonic transducer is excited, by an electrical impulse, there is a tendency for the piezoelectric material to continue oscillating or to "ring". However, to be of diagnostic value, a transducer must be well damped so as to produce short pulses which are needed to resolve fine structures in the body. This ability is called axial resolution. The axial resolution is proportional to the resonant frequency of the transducer. The higher the resonant frequency, the shorter the ultrasonic pulse and the better the axial resolution.
To increase the efficiency of the transducer, a quarter wavelength matching material is often applied to the front face of the piezoelectric element to match the high impedance of the piezoelectric element to the lower impedance of the tissues to be probed. Optimum matching is obtained by making the impedance of a quarter wave matching layer equal to the geometric mean of the impedance of the piezoelectric element and the impedance of the tissue. Transducers using quarter wave matching layers have a high sensitivity because most of the ultrasonic energy is radiated in the required forward direction and the received ultrasonic energy is efficiently coupled back o the piezoelectric element. Such transducers usually employ only a light backing (of relatively low acoustic impedance) which provides mechanical support for the piezoelectric element.
In addition to axial resolution, it is important that transducers be capable of transmitting to and receiving ultrasonic energy from tissues deep in the body so that deep lying structures can be examined. The ability to examiner these deep lying structures is called penetration. However, the attenuation of ultrasonic waves increases sharply with increases in frequency. Thus, a compromise must be found between good resolution and acceptable penetration.
In general, different frequencies of operation are used for different application:. In medical applications, the adult heart is examined at 3 Megahertz, a child's heart at 5 Megahertz and frequencies of 10-15 Megahertz are typical for examination of the adult eye.
During the ultrasonic examination of organs, it is often necessary to first examine with a high degree of penetration (for example to obtain an overall view of an organ) and then to look more closely at an area of interest with a higher resolution. Transducers which operate at single frequencies must be changed in the course of a procedure. This is difficult as it requires that the procedure be interrupted, equipment readjusted, the area of interest located again and the procedure resumed. If the area of interest within the body were to move, it would be difficult to find again with the high resolution transducer and thus the change in transducer would be effectively useless.
Under such circumstances, it is beneficial to be able to operate the transducer effectively at more than one frequency. The benefits of operating a transducer at more than one frequency are recognized in the art and are also useful in other applications of ultrasound such as in the field of non-destructive testing. A number of methods have been proposed to create a multiple frequency ultrasonic transducer. These are based o: laminating layers of piezoelectric ceramics of various thickness. A variety of operating frequencies can be obtain d by exciting one or more layers of the piezoelectric ceramics.
An example of such a transducer system is disclosed in U.S. Pat. No. 4,276,491 to Daniel entitled Focusing Piezoelectric Ultrasound Medical Diagnostic System, issued to the assignee of the present invention. Daniel discloses an ultrasonic piezoelectric transducer consisting of two separate transducer elements bonded together. By switching the electrodes which are connected to the transmit and receive circuits of an ultrasonic medical scanning apparatus, the single transducer may be used for imaging with high resolution at one frequency and for pulse Doppler velocity measurements at a different frequency. The two elements are driven in series for Doppler measurements and only the front element is used for pulse echo measurements. While quite satisfactory in terms of achieving its intended results, this approach does not make maximum use of the available piezoelectric elements to achieve maximum transducer sensitivity.
Composite materials have been developed to improve the characteristics of single phase material. The goal of such developments is to comb ne the desirable properties of two different constituent materials to produce a composite with superior characteristics. In piezoelectric composite materials, the desired properties of high electro-mechanical coupling coefficient k.sub.t and low acoustic impedance z has been achieved by combining a piezoelectric ceramic and a passive polymer. This composite structure, formally characterized by 1-3 connectivity consists of a two-dimensional arrays of parallel piezoelectric ceramic rods embedded in a polymer matric, as described in Composite Piezoelectric Transducers R. E. Newnham, et al., Materials in Engineering, Vol. 2, Dec., 1980, pp 3-106)
A considerable variety of properties can be achieved with these composites by varying the piezoelectric ceramic and polymer components, their volume fractions and the lateral spatial scale of the structure. The lateral spatial scale is determined by the relative width of the ceramic rod and the polymer gap. When the composite vibrates, the resonance characteristic of the composite is much more complex than that of a single phase ceramic disc.
The resonance characteristics of a ceramic disc are essentially a single fundamental resonant and a single fundamental anti-resonant frequency. The resonance frequency (fr) is inversely proportional to the thickness of the ceramic disc and is given by: EQU fr=Nt/.sub.t
Where:
Nt=the frequency constant of the ceramic material; and PA1 t=thickness of the ceramic disc.
The resonance modes of a composite material are much more complex. While the complex nature of these resonances has been known, there has been no attempt to exploit the multifrequency characteristics of composite materials to produce a multifrequency transducer. This is probably due to the complex interaction between the modes of vibration, which results in mode which do not produce a "clean" response at a desired frequency. If such clean modes of vibration were produced, a composite transducer could be used, with an appropriate electronic system, for ultrasonic imaging at at least two different frequencies.